The present invention relates to a hydraulic control system for a continuously variable traction roller transmission.
One of hydraulic control systems is disclosed, for example, in JP-U 63-92859. In this reference, a continuously variable traction roller transmission includes an input disc, an output disc, a pair of traction rollers disposed between a toroidal concavity defined by the two discs for frictional engagement therewith, and a pair of roller support members for rotatably supporting the traction rollers. Each of the roller support members being rotatable on a rotation shank which crosses a rotation axis of the two discs at right angles, and movable in an axial direction of the rotation shank. The transmission further includes a hydraulic cylinder apparatus for urging the roller support members to move in the axial direction of the rotation shank, a shift control valve for controlling hydraulic pressure to be supplied to the hydraulic cylinder apparatus, and a cam for transmitting rotational motion of the roller support members to the shift control valve. Inclination of each traction roller is adjusted by regulating an axial position of that roller support member by the hydraulic cylinder apparatus, thus continuously changing the gear ratio established between the input and output discs. A part of hydraulic fluid to be supplied to the hydraulic cylinder apparatus is always discharged through a hydraulic passage assuming orifice effect. Discharged hydraulic fluid is used for lubrication of frictional portions between the traction rollers and the two discs. Constantly discharging hydraulic fluid within the hydraulic cylinder apparatus through the orifice provides a damping effect upon shifting. That is, when the shift control valve operates based on a quick shifting command, hydraulic pressure within the hydraulic cylinder apparatus changes quickly, and a piston thereof responds quickly to this change, resulting in rotation of the roller support members. Since this rotation is fed back to the shift control valve through the cam, hunting may occur unless the hydraulic control system assumes a damping effect. With this transmission system, a part of hydraulic fluid within the hydraulic cylinder apparatus is always discharged as described above, thereby creating a slow rate of change of the hydraulic pressure within the hydraulic cylinder apparatus, and preventing the occurrence of hunting.
With such a hydraulic control system for a continuously variable traction roller transmission, however, since a part of hydraulic fluid within the hydraulic cylinder apparatus is always discharged, high pressure hydraulic fluid is consumed in quantity, resulting in a shortage of pump capacity. If the pump size is increased to secure the required oil quantity, the entire transmission will have to be enlarged in size.